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Weight & Balance

Introduction:

  • Weight and balance calculations determine the Center of Gravity (C.G.) of the airplane to ensure it is safe for flight
  • Position of the center of gravity is affected not only by the total but also the distribution of weight throughout the aircraft
  • Weight and balance is directly relates to the stability of the aircraft
  • Instructions and examples can be found in the aircraft manual under section 6 for your specific aircraft
  • Weight and balance data is required to satisfy the requirements of FAR 21.5

Center of Gravity:

  • The C.G. must always be within limits, however, depending where in the allowable range the C.G. falls will effect performance
  • The center of gravity is the theoretical point where the entire weight of the airplane is considered to be concentrated
  • Forward CG:
    • Stable feeling
    • Nose Heavy
    • Longer takeoff distance (more airflow required to provide more force to lift heavy nose)
    • High stall speeds (more airflow deflection of the elevator required to maintain altitude at slower airspeeds resulting in high AOAs)
  • Rearward CG:
    • Less stable
    • Tail heavy
    • Hard to recover from stall or spin
    • Higher true airspeed
    • More tail down force

Overweight Aircraft:

  • Most aircraft will never be too light to fly however overweight aircraft pose very serious safety threats
  • People like R&B singer Aaliyah have died when pilots neglect to complete a proper preflight
  • Limitations:
    • Longer takeoff run
    • Higher takeoff speed
    • Reduced angle and rate of climb
    • Reduced cruising speed
    • Shorter range
    • Higher stalling speed
    • Longer landing roll

Definitions:

  • Center of Gravity: imaginary point where the aircraft would balance if suspended
  • CG Limits: the forward and aft center of gravity locations for a given weight
  • Reference Datum: imaginary vertical plane from which all horizontal distances are measured (firewall, leading edge, etc.)
  • Basic Empty Weight (BEW): weight of standard airplane, optional equipment, unusable fuel, and full operating fluids, including full engine oil. Any changes must be documented
  • Unusable Fuel: fuel that cannot be drained
  • Licensed Empty Weight: like BEW, but does not count full engine oil, only undrainable oil
  • Ramp Weight: airplane loaded for flight prior to engine start
  • Gross Takeoff Weight: weight of the airplane just before brake release to begin takeoff roll
  • Gross Landing Weight: takeoff weight minus the fuel burned en-route
  • Zero Fuel Weight: weight of the aircraft before addition of fuel
  • Payload: weight of only the passengers, baggage, and cargo
  • Useful Load: weight of crew and usable fuel
  • Maximum Ramp Weight: max weight for ground operations
  • Maximum Takeoff Weight: max weight for takeoff
  • Maximum Landing Weight: max weight for landing based on stress of impact on gear
  • Usable Fuel: fuel available for flight
  • Arm: distance from the datum
  • Moment: measurement of the tendency of the weight to cause rotation at the fulcrum
  • Loading Graph: used to find the moment for loads in the airplane
  • Center of Gravity Moment Envelope: shows limits with proposed loading

Weight and Balance Data
Figure 1: Weight and Balance Data

Procedure:

  • Block 1: Determine the Basic Empty Weight (BEW) of the airplane (found in POH)
  • Block 2: Determine the basic empty weight moment of the airplane (found in POH)
  • Block 3: Determine the weight of the pilot and passenger
  • Block 4: Determine the moment of the pilot and passenger (weight x arm = moment)
  • Block 5: Determine the weight of the rear passengers
  • Block 6: Determine the moment of the rear passengers (weight x arm = moment)
  • Block 7: Determine the weight of the baggage
  • Block 8: Determine the moment of the baggage (weight x arm = moment)
  • Block 9: Determine the weight of the baggage as in step 7
  • Block 10: Determine the moment of the baggage as in step 8, using the new arm
  • Block 11: Add all weights together to get the Zero Fuel Weight (Z.F.W.)
  • Block 12: Add all moments together
  • Block 13: Determine the weight of the ramp fuel
  • Block 14: Determine the moment of the ramp fuel (weight x arm = moment)
  • Block 15: Determine the ramp weight (Z.F.W. + Ramp Fuel)
  • Block 16: Determine the ramp moment (Z.F.W. moment + Ramp Fuel moment)
  • Block 17: Subtract taxi fuel used (~8 lbs)
  • Block 18: Subtract taxi fuel moment (~384)
  • Block 19: Add Z.F.W. and Ramp Weight together, then subtract Taxi Fuel to get the Gross Takeoff Weight (G.T.W.)
  • Block 20: Add Z.F.W. moment and Ramp Weight moment together, then subtract Taxi Fuel moment
  • Block 21: Estimate trip fuel weight
  • Block 22: Determine the moment of the trip fuel (weight x arm = moment)
  • Block 23: Subtract trip fuel weight from G.T.W. to get the Gross Landing Weight (G.L.W.)
  • Block 24: Subtract trip fuel moment from G.T.W. moment
  • Block 25: Divide block 12 by block 11
  • Block 26: Divide block 20 by block 19
  • Block 27: Divide block 24 by block 23
  • Block 28: Determine maneuvering speed (Va)

Weight Shift Formula

  • If you shift weight after determining the aircrafts weight and balance then verify your calculations with the weight shift formula

Weight Moved = Distance C.G.Moves
Aircraft Weight Distance Between Arm Locations

Case Studies:

  • National Transportation Safety Board Identification: ANC13FA091: The NTSB determines the probable cause(s) of this accident to be:
    • The pilot's improper decision to load the airplane beyond its allowable takeoff weight and center of gravity limits, which resulted in a loss of control during the initial climb. Contributing to the accident was the external load and the downwind takeoff
  • National Transportation Safety Board Identification: ERA14LA450: The NTSB determines the probable cause(s) of this accident as follows:
    • The pilot's inadequate preflight planning, which resulted in a takeoff with the airplane's center of gravity aft of its limit and led the airplane to exceed its critical angle of attack and experience an aerodynamic stall during the initial climb. Contributing to the accident was the pilot's lack of flight experience in the aircraft make and model
  • National Transportation Safety Board Identification: ERA14CA408: The NTSB determines the probable cause(s) of this accident as follows:
    • The pilot/owner/builder's improper weight and balance calculations, which rendered the airplane uncontrollable in the pitch axis
  • National Transportation Safety Board identification: ERA14FA343: The NTSB determines the probable cause(s) of this accident as follows:
    • The pilot’s failure to secure the cargo in the cargo compartment, which resulted in a weight shift that led to the center of gravity exceeding its aft limit during a go-around attempt and a subsequent aerodynamic stall. Also causal to the accident were the pilot’s inadequate preflight inspection and his loading the airplane beyond the cargo compartment weight limit
  • The National Transportation Safety Board Identification: CEN13IA563: The NTSB determines the probable cause(s) of this incident as follows:
    • The pilot’s improper weight and balance calculations, which resulted in the airplane exceeding its weight and center-of-gravity limits and led to a loss of pitch control during takeoff, and the operator’s failure to obtain required weight information and to ensure that the flight was properly loaded

Conclusion:

  • Weight and balance is one of the easiest subjects to become complacent with over time but referencing our case studies, its always important
  • The weight and balance document is the "W" in the acronym "ARROW," which helps us remember the documents required for flight

References: